This invention relates in general to scanned display systems and in particular to those utilizing cathode ray tubes such as television receivers.
Among the geometric distortions produced when a beam emanating from a theoretical point source is caused, by synchronous deflection along two axes, to scan a viewing screen having a radius of curvature greater than the center screen-to-point source distance, is pincushion. Pincushion is readily observable when information in the form of a graticule is displayed, appearing as a "bowing in" of the reproduced image. If the viewing screen has a greater radius of curvature along both axes (as is the case in present day television receivers), pincushion results along both axes, that is, along horizontal and vertical lines. The former is called "top and bottom" pincushion and the latter "side" pincushion. Quite obviously pincushion distortion is maximized when the radius of the screen curvature is infinite (i.e., flat screen), and while cathode ray tubes generally do not have flat screens, aesthetic considerations usually dictate that nearly flat screens be used.
Of particular interest in the present invention is the top and bottom pincushion or "vertical sag" as it is often called. Analysis of the scanning process typically used in cathode ray display systems shows that a high frequency horizontal deflection system causes side-to-side scanning of the viewing screen while a lower frequency vertical deflection system causes successive side-to-side scans to progress in a downward direction.
It is well known that correction of top and bottom pincushion distortion may be accomplished by adding an appropriate horizontal frequency deflection component to the normal vertical deflection signal. It is also well known that top and bottom pincushion is zero for the mid-screen horizontal scan line and increases progressively (i.e., greater sagging) with increased vertical deflection angle. As a result, the correction signal is required to vary in amplitude from a maximum at one polarity, corresponding to horizontal scan lines at the top of the viewing screen through zero, corresponding to those at mid-screen, to a maximum at the opposite polarity corresponding to the bottom of the screen. In addition to the amplitude variations described, the horizontal rate signal should, for ideal correction, be of such character that its effect upon the vertical deflection is the complement of the distortion. Such a waveform is quite complex and difficult to fabricate and general practice is to approximate the ideal correction in the form of either a cosine, parabolic, or sine squared waveform.
Top and bottom pincushion compensation systems may be categorized as being either high level or low level, the former characterized by direct yoke current correction and the latter by addition of a correction signal to the vertical deflection amplifier. High level correction uses a saturable reactor in series with the vertical deflection yoke. Horizontal rate signals are applied to balanced inputs of the reactor, to which is also applied a sample of the vertical deflection signal. The reactor is wound such that the amplitude and polarity of the induced horizontal component coupled to the vertical deflection yoke is determined by the instantaneous polarity and amplitude of the applied vertical signal.
With the increased deflection current required by lower impedance yokes such as those having toroidal wound coils, the saturable reactor, which must carry the entire vertical scanning current, becomes a prohibitively large, wasteful and expensive device and a very inefficient mechanism for correcting pincushion distortion. As a result the alternative low level systems are currently enjoying increased attention from display system manufacturers.
As mentioned, a low level correction system differs from the above-described high level system in that the appropriate correction waveform is produced by specialized receiver circuitry at a relatively small amplitude and applied to an appropriate point in the vertical deflection amplifier configuration rather than directly to the "high power" yoke. The low level signal generated comprises horizontal frequency sine-wave signals having an envelope defining a first maximum amplitude at the beginning of vertical scan, a linearly decreasing-to-zero amplitude at mid-scan followed by a linearly increasing-to-a-second-maximum amplitude at the end of vertical scan, giving the waveform envelope a bow-tie shape.
One simple method of generating this "bow-tie" signal is described in application Ser. No. 520,837 filed Nov. 4, 1974 in the name of Stanley Lehnert and assigned to the assignee of the present invention in which a full wave rectified vertical scan signal is serrated by horizontal sweep retrace signals to form a train of pulses. A resonant tank circuit is stimulated by the pulse train to produce the "bow-tie" waveform.
Regardless of the means used to generate the bow-tie signal, low level systems place an additional burden on the vertical deflection amplifier by the imposition of the correction signal upon the vertical scan signal. The problem is made more acute by the fact that the vertical yoke has inductive reactance and, therefore, presents a substantially increased impedance to the higher frequency correction components than to the vertical scan signals. The different impedances of the vertical yoke for vertical scanning and horizontal rate correction signals means that even small pincushion currents require disproportionately large voltage swings by the vertical deflection amplifier. For example, a typical vertical deflection yoke, having series connected windings, requires approximately 10 volts of vertical scan signal to produce 1 ampere of deflection current. The same yoke requires approximately 80 volts of horizontal frequency correction signal to produce the typically desired 35 milliamperes of pincushion correction current.
In contemporary television receiver design, the vertical deflection amplifier is "transistorized" and the supply voltage, as well as the type of output transistors used, are subject to stringent cost limitations. This means that typical vertical deflection amplifiers, while adequate to fulfill receiver scan requirements, require a substantial increase in operating voltages and more expensive output devices to accommodate the increased voltage swing produced by the imposed correction signal.
The costs associated with vertical amplifier changes have militated against low level pincushion correction systems. The present invention overcomes this limitation of prior art systems.